This Element surveys contemporary philosophical and psychological work on various forms of irrationality: akrasia, strange beliefs, and implicit bias. It takes up several questions in an effort to better illuminate these more maligned aspects of human behaviour and cognition: what is rationality? Why is it irrational to act against one's better judgement? Could it ever be rational to do so? What's going wrong with beliefs in conspiracy theories, those arising from self-deception, or those which are classed as delusional? Might some of them in fact be appropriate responses to evidence? Are implicit biases irrational when they conflict with our avowed beliefs? Or might they be rational insofar as they track social realities?